A Glen of Imaal Terrier is a powerful animal. It has behaviour patterns that have been genetically implanted by centuries of heritage. It means it does not understand political correctness. It has not watched Animal Hospital nor listened to modern vets as to how it should behave. It knows, so it is.
The Glen temperament is self reliance plus an ability to cope & this needs to be fully understood before ownership of a Glen is embarked upon. A Glen was never intended, or desired, to have the spark & fire of its Irish cousins. It was desired to be calm and peaceful until called upon & this causes huge confusion with some people. When their gentle peaceful Glen, for the first time in its life, answers an insult with bared teeth they immediately consider it aggressive. It is not aggressive, schizophrenic, psychotic, crazy, or necessarily "vicious"; it has been called upon and it has answered.
It is your responsibility to respond correctly. Let your Glen get away with it & he will perceive his own ranking in the pack hierarchy to be higher than yours & he will continue to challenge. A Glen that meets every other dog, in every situation, with a roar of fury is a liability. Because he has proven his dominance over you he believes he can dominate the world. If you don’t think you will be able to deal with it before, during and after don’t have a Glen.
Glen of Imaal Terriers need to know the basics of good behaviour. Their name, coming and waiting are things relatively easy to teach. Your Glen will also teach you to wait until he is ready to do these things but a compromise can generally be worked out!
Some Glens have done quite serious training and the number with the Kennel Club Good Citizen Test (Bronze) grows yearly. Some have gone on to do the Silver. Good manners maketh the man and they also maketh the dog.